Portable spectrophotometer and method for characterising solar collector tubes

Abstract

Portable spectrophotometer and method for characterizing solar collector tubes for simultaneously and on-field characterizing reflection and transmission coefficients. This device includes all the components needed to take this measurement, such as a module that takes the measurement of the reflection coefficient (R) of the inner tube ( 1′ ), a module that takes the measurement of transmission coefficient (T) of the outer tube ( 1″ ), an electronic data acquisition and processing system ( 12 ), an external computer ( 13 ) for controlling the device and sending the measured data ( 17 ) and a communication system ( 15 ) between device and the computer ( 13 ).

The invention claimed is: 
     
       1. A portable spectrophotometer for characterizing solar collector tubes formed by an inner tube and an outer tube, comprising:
 a reflection module for measuring the reflection coefficient of the inner tube; 
 a transmission module for measuring the transmission coefficient of the outer tube; 
 an electronic data acquisition and processing system; 
 an external computer for controlling the spectrophotometer and sending the measured data; 
 wherein each of the transmission and reflection modules comprises at least one light emitting diode as an optical source for each wavelength to be measured from solar spectrum and at least one photodetector sensitive to the wavelength for obtaining a transmission signal for the outer tube and a reflection signal for the inner tube and at least one reference photodetector placed after the at least one light emitting diode by which a reference signal of the at least one light emitting diode is obtained for transmission or for reflection. 
 
     
     
       2. The portable spectrophotometer according to  claim 1 , wherein the number of light emitting diodes installed as well as the wavelengths to which the sweeping is performed are elected by virtue of the resolution and range required for characterizing each collector tube. 
     
     
       3. The portable spectrophotometer according to  claim 1 , wherein the number of light emitting diodes is between 6 and 24 and within the spectral range between 300 and 2500 nm corresponding to the solar spectrum. 
     
     
       4. The portable spectrophotometer according to  claim 1 , wherein two light emitting diodes are used for each wavelength to be measured and reflection and transmission coefficients are simultaneously obtained for the inner tube and outer tube, respectively. 
     
     
       5. The portable spectrophotometer according to  claim 1 , wherein the data acquisition system consists of two amplification stages for each photodetector. 
     
     
       6. The portable spectrophotometer according to  claim 5 , wherein at least one of the two amplification stages varies its gain by software commands. 
     
     
       7. The portable spectrophotometer according to  claim 1  further includes a DSP-type card with a signal processing system of the type lock-in. 
     
     
       8. The portable spectrophotometer according to  claim 7 , wherein the signal processing lock-in and the signal performing the processing in order to modulate the LEDs acting as optical sources is implemented using the same DSP-type card. 
     
     
       9. The portable spectrophotometer according to  claim 1 , further comprises a case that protects the optical and electronic components and enables the portability and allows it to be quickly and easily coupled to the collector tube desired to be measured. 
     
     
       10. The portable spectrophotometer according to  claim 1 , wherein signal from the at least one LED is modulated by sinusoidally varying the supply current of the LEDs light to remove the influence of ambient light. 
     
     
       11. A method for characterizing solar collector tubes by using the portable spectrophotometer of  claim 1 , wherein the measurement of reflection and transmission coefficients of the tubes comprises the following steps:
 positioning the spectrophotometer so that it is stably supported on the collector tube; 
 sequentially turning on and off the different light emitting diodes of the spectrophotometer, while measuring the signal received by the corresponding photodetectors; 
 normalizing the data obtained in reflection and transmission photodetectors with their respective reference measurements in order to eliminate the influence of variations in the intensity caused by the light emitting diodes; 
 subsequently, obtaining the transmission coefficient of the outer tube by relating the obtained normalized transmission value with that obtained when measuring a known pattern; 
 obtaining the reflection coefficient of the inner tube by deducting the effect of crossing twice the outer tube, dividing by the square of the previously calculated transmission coefficient with an necessary angle correction, and obtaining the final value of the coefficient by making reference to a known pattern; 
 wherein the values corresponding to the known pattern are stored in the spectrophotometer after a prior calibration, which requires the use of a tube with known reflection coefficients with the transmission calibrated at air unity value, wherein the calibration is carried out following the first three steps of this same procedure. 
 
     
     
       12. The method of  claim 11 , wherein signal from the at least one LED is modulated by sinusoidally varying the supply current of the LEDs light to remove the influence of ambient light.